Edward Kmett ekmett at
Fri Aug 21 04:25:08 UTC 2015

When (ab)using them for this purpose, SmallArrayArray's would be very handy
as well.

Consider right now if I have something like an order-maintenance structure
I have:

data Upper s = Upper {-# UNPACK #-} !(MutableByteArray s) {-# UNPACK #-}
!(MutVar s (Upper s)) {-# UNPACK #-} !(MutVar s (Upper s))

data Lower s = Lower {-# UNPACK #-} !(MutVar s (Upper s)) {-# UNPACK #-}
!(MutableByteArray s) {-# UNPACK #-} !(MutVar s (Lower s)) {-# UNPACK #-}
!(MutVar s (Lower s))

The former contains, logically, a mutable integer and two pointers, one for
forward and one for backwards. The latter is basically the same thing with
a mutable reference up pointing at the structure above.

On the heap this is an object that points to a structure for the bytearray,
and points to another structure for each mutvar which each point to the
other 'Upper' structure. So there is a level of indirection smeared over

So this is a pair of doubly linked lists with an upward link from the
structure below to the structure above.

Converted into ArrayArray#s I'd get

data Upper s = Upper (MutableArrayArray# s)

w/ the first slot being a pointer to a MutableByteArray#, and the next 2
slots pointing to the previous and next previous objects, represented just
as their MutableArrayArray#s. I can use sameMutableArrayArray# on these for
object identity, which lets me check for the ends of the lists by tying
things back on themselves.

and below that

data Lower s = Lower (MutableArrayArray# s)

is similar, with an extra MutableArrayArray slot pointing up to an upper

I can then write a handful of combinators for getting out the slots in
question, while it has gained a level of indirection between the wrapper to
put it in * and the MutableArrayArray# s in #, that one can be basically
erased by ghc.

Unlike before I don't have several separate objects on the heap for each
thing. I only have 2 now. The MutableArrayArray# for the object itself, and
the MutableByteArray# that it references to carry around the mutable int.

The only pain points are

1.) the aforementioned limitation that currently prevents me from stuffing
normal boxed data through a SmallArray or Array into an ArrayArray leaving
me in a little ghetto disconnected from the rest of Haskell,


2.) the lack of SmallArrayArray's, which could let us avoid the card
marking overhead. These objects are all small, 3-4 pointers wide. Card
marking doesn't help.

Alternately I could just try to do really evil things and convert the whole
mess to SmallArrays and then figure out how to unsafeCoerce my way to
glory, stuffing the #'d references to the other arrays directly into the
SmallArray as slots, removing the limitation  we see here by aping the
MutableArrayArray# s API, but that gets really really dangerous!

I'm pretty much willing to sacrifice almost anything on the altar of speed
here, but I'd like to be able to let the GC move them and collect them
which rules out simpler Ptr and Addr based solutions.


On Thu, Aug 20, 2015 at 9:01 PM, Manuel M T Chakravarty <
chak at> wrote:

> That’s an interesting idea.
> Manuel
> > Edward Kmett <ekmett at>:
> >
> > Would it be possible to add unsafe primops to add Array# and SmallArray#
> entries to an ArrayArray#? The fact that the ArrayArray# entries are all
> directly unlifted avoiding a level of indirection for the containing
> structure is amazing, but I can only currently use it if my leaf level data
> can be 100% unboxed and distributed among ByteArray#s. It'd be nice to be
> able to have the ability to put SmallArray# a stuff down at the leaves to
> hold lifted contents.
> >
> > I accept fully that if I name the wrong type when I go to access one of
> the fields it'll lie to me, but I suppose it'd do that if i tried to use
> one of the members that held a nested ArrayArray# as a ByteArray# anyways,
> so it isn't like there is a safety story preventing this.
> >
> > I've been hunting for ways to try to kill the indirection problems I get
> with Haskell and mutable structures, and I could shoehorn a number of them
> into ArrayArrays if this worked.
> >
> > Right now I'm stuck paying for 2 or 3 levels of unnecessary indirection
> compared to c/java and this could reduce that pain to just 1 level of
> unnecessary indirection.
> >
> > -Edward
> > _______________________________________________
> > ghc-devs mailing list
> > ghc-devs at
> >
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